1. Field of the Invention
The present invention relates to an image processing device, an image processing method, and a medium.
2. Description of Related Art
Some image forming apparatuses can print posters by dividing a whole image of one page into images on multiple sheets. The sheets, each of which has a gluing margin, are joined to each other with the gluing margins laid on top of each other for a large image to be obtained.
A region of gluing margins laid on top of each other has light transmittance and reflectance different from a region of a single sheet, causing an image formed across the gluing margin region to be observed as an image with discontinuous densities.
A conventional method controls the density of an image to be formed on a gluing margin of each sheet so that an image on the gluing margins laid on top of each other has a target density. (See Japanese Unexamined Patent Application Publication No. 2004-82610.)
Unfortunately, when an image is formed only on an upper one of superposed sheets to reduce toner consumption, such a conventional density control cannot be applied.
When an image is formed with screen patterns by dithering for halftone reproduction, gradation conversion is repeated in dither matrix units. A dither matrix across the boundary between a gluing margin and a region other than the margin causes conspicuous density discontinuity due to the above-mentioned difference in light transmittance and reflectance.
The size of the dither matrix can be adjusted to the boundary of the margin. (See Japanese Unexamined Patent Application Publication No. 2011-23964.) Unfortunately, it takes time to adjust the size one by one depending on an image to be formed.